1. Field of the Invention
The present field of the invention relates to electroluminescent lamps, and more particularly to a method for manufacturing water resistant electroluminescent lamps that are suitable for many low-cost consumer applications.
2. Description of the Prior Art
Conventional electroluminescent (EL) lamp manufacturing techniques may be divided into two basic processes. The first, a screen-printing process in which a lamp is constructed layer by layer. More particularly, the lamp is constructed using the steps of making indium tin oxide (ITO) plated plastic film; applying EL phosphor ink on the ITO plating to form lighted areas; applying capacitive dielectric ink over the EL phosphor ink; applying electrically conductive ink over the capacitive dielectric ink to form a second capacitive plate; applying electrically conductive ink over the ITO plated plastic film outboard of the capacitive dielectric ink layer to provide a front capacitive electrode connection. This first construction must then be protected from environmental attack via the means of either an encapsulating lamination, or secondarily by application of a water repellant electrical insulating coating containing an ultraviolet light activated polymer.
The screen-printing process allows intricate graphics effects to be created using relatively simple manufacturing processes. However, screen-printed EL lamps having high luminance or superior electrical characteristics tend to be costly to manufacture. This is in part due to the difficulty of precisely aligning all conductive, insulating and light emissive layers when processing is performed with typical screen-printing methods. Such layer-to-layer alignment difficulties can result in decreased production yields, especially in applications where there is limited space to provide electrical clearance between the rear electrode and front electrode connection described above.
The second common process is a laminated EL lamp assembly. In this process, a first film, which supports a metal foil, is passed below a metering roller or blade that applies an insulating layer of capacitive dielectric ink. A second, light transmissive ITO plated film is similarly passed below a roller or blade, which applies a layer of EL phosphor ink onto the ITO plating. In order to achieve both a uniform light output and reliable electrical characteristics, the thickness of the insulating dielectric and phosphor layers must be precisely controlled, along with the grain dispersion of the EL phosphor particles within the phosphor layer. Thus this typically continuous lamination requires very tight control over both ink rheology and ink application processes.
Once the ink layers have dried and been inspected for defective areas, the first and second films are laminated together to form an EL lamp core. This film lamination method requires heat and/or pressure, which must be tightly controlled so that the light and electrical characteristics of the finished lamp are consistent. Additionally, since the EL phosphor layer is sensitive to water contamination, once the finished lamp is cut into usable shape and size, then electrically terminated, it is then encapsulated within a moisture resistant lamination film (such as Allied Signal""s xe2x80x9cACLARxe2x80x9d CTFE).
The continuous lamination method produces foil EL lamps, which are high performance, high priced lamps typically unsuitable for complex graphics or other price sensitive applications. Laminated foil EL lamps are also typically thicker and less flexible than screen printed lamps, limiting their application to those where flexibility and thickness are of less concern.
In both of the above methods, metal and metal oxides are plated upon a plastic carrier film that is typically used as the basis material for the front conductive layer. The usual film of choice is polyester plastic film plated with indium tin oxide (ITO). This particular plating exhibits the additional construction weakness of fracturing under close bend radius flexing. These fractures have been demonstrated to cause both dimmed areas, and even total non-illumination of EL lamps of these constructions, due to the interrupted current path at the location of breakage.
In U.S. Pat. No. 5,667,417, of which, this is an improvement of William Stevenson (one of the inventors here) teaches a method of producing low cost metal foil based EL lamps of potentially complex graphic pattern by using a precise indexing system that applies well known flexible circuit technology to a cost-effective continuous production process. In this patent, a plurality of sprocket holes allows a precise indexing and positioning of the EL lamp pattern relative to the screen-print ink applicator. This method ensures that minimal electrical clearances may be maintained, due in full to the precision preparation of metal foil electrodes relative to the indexing sprocket holes.
A weakness of this method is that the sprocket holes may show wear during multiple print layer passes by the effect of drive pressures applied to the indexing holes"" leading edges during transport advance. This wear factor can contribute to print misalignment of subsequent layers, and the resulting decrease in production yield. Additionally, the space consumed by sprocket holes reduces the usable area of raw material film, and thus further limits total production yield. A further weakness shown by this process is that presently available light transmissive, electrically conductive coatings and inks have limited utility due to their inherent high resistance, when compared to traditional conductive metal and metal oxide plating over plastic film.
Accordingly, there is a need for an improved indexing means that results in increased EL lamp production accuracy and product performance versus material yield.
The present invention is directed to a method of manufacturing EL lamps incorporating some of the processes used in the manufacture of flexible printed circuit boards.
In an exemplary embodiment of the invention, the method of the present invention includes the following steps. In the first step, a process basis carrier film having metal foil bonded to its surface is prepared for further process by die cutting or chemically etching the desired rear capacitive electrodes that precisely define illuminated areas, power input distribution elements and associated electrical contacts, and optical registration indicia. Following this, the carrier film is placed onto a commercially available transport system that incorporates an optical registration system to precisely position the image area for each print cycle. This method allows the precise (+/xe2x88x92 less than 0.002xe2x80x3 in X, Y and xcex8 axis) physical positioning of the basis carrier film without deleterious effect upon the positioning reference means. Using this method allows practically unlimited numbers of print layers to be applied without concern for layer-to-layer alignment.
In the third step, a layer of hygrophobically compounded capacitive dielectric ink is screen-printed in a pattern that completely overlaps the rear capacitive electrode foil. Through precise, optically registered positioning the capacitive dielectric ink is allowed minimal bleed past the rear electrode edges.
The fourth step consists of printing a high dielectric, hygrophobically compounded EL phosphor ink to further define the illuminated area. Again, precise optically registered positioning of the basis carrier film allows limited ink bleed beyond the rear electrode element. Following this, in the fifth step a layer of hygrophobically compounded electrically conductive, light transmissive ink is applied to cover the EL phosphor layer, forming a front capacitive electrode. The front electrode layer ink is allowed to bleed beyond the EL phosphor layer in order to make contact with a metal foil power conductor.
Next, in step six a transparent polyester film or ultraviolet activated dielectric coating is applied to the entire surface of the lamp. Openings in this layer may be made allowing exposure of the metal foil layer precisely defining electrical power contact areas. Following this step, the completed EL lamp is then cut from the basis carrier film.
A first embodiment of an EL lamp manufactured by the method of the present invention comprises a rear capacitive electrode that precisely defines the area of illumination that is bonded to either a plastic or paper core stock. A layer of capacitive dielectric is applied over the rear capacitive electrode, providing electrical isolation between the rear capacitive electrode and the overlying next printed layer of EL phosphor ink. Further a layer of translucent conductive ink is applied over the EL phosphor ink layer creating a capacitive front electrode that is protected by an insulating transparent polyester film layer.
In a second embodiment of the an EL lamp manufactured by the method of the proposed invention, the capacitive electrodes, capacitive dielectric and EL phosphor ink layers are bonded to both surfaces of the of the plastic or paper core stock. This embodiment provides a low-cost high quality and performance EL lamp that emits light from both surfaces.
The method of the present invention provides the ability to manufacture EL lamps at a cost fractional of that of comparable conventional construction. Additionally, these lower-cost EL lamps can be manufactured on readily obtainable automated production equipment.
Further features and advantages of the present invention will be appreciated by a review of the following detailed description when taken in conjunction with the following drawings.